It is possible, using recombinant DNA technology, to clone and express in bacteria and yeast a variety of genes which are not present in such organisms as they occur in nature. For example, procedures for cloning DNA segments in E. coli, by inserting the DNA into or bacteriophage genome, are well established and frequently used to isolate prokaryotic and eukaryotic genes.
The yeast Saccharomyces cerevisiae (S. cerevisiae) is also used as a host for heterologous gene expression and protein secretion. Availability of techniques which make it possible to introduce exogenous DNA into yeast genomic DNA has made it possible to develop yeast strains which produce and secrete foreign proteins such as alpha interferon, epidermal growth factor, calf prochymosin and beta-endorphin. Heterologous eukaryotic genes can be expressed in S. cerevisiae if they are placed under the control of a yeast gene promoter region. Heterologous protein yields will be determined, at least in part, by the promoter chosen. As an example, the promoter from the yeast gene for phosphoglycerate kinase (PGK) has been described as directing expression of heterologous genes with efficiency at least 500-fold greater than that evident when the TRP1 promoter is used. Mellor, J. et al., Gene, 33:215-226 (1985). Hitzeman and co-workers describe work to increase expression of the gene encoding bovine growth hormone in yeast. The 5'-promoter region, translation signal and signal peptide sequences were replaced with yeast genomic DNA from similar regions. Hitzeman, R. A. et al., Nature, 295:717-722 (1981).
The type of vector chosen (e.g., a high-copy-number 2 micron plasmid-based shuttle vector, rather than an unstable high-copy-number ARS-based plasmid or a low-copy-number stable ARS/CEN plasmid) also affects yields of heterologous proteins.
The availability of yeast promoters which are efficient in directing expression of a heterologous gene and whose activity can be regulated would be desirable.